Insulated conductor and method of making same



July 25, 1961 w. 1 MARSHALL 2,993,820

INSULATED CONDUCTOR AND METHOD OF MAKING SAME Filed April 9, 1956 /2 F i. /z

fnl/.en tor: h/f/ter L. Marsha/A /7/'5 A ttorney United States Patent O 2,993,820 INSUL'ATED 'CONDUCTOR AND METHOD 0F MAKING SAME Walter L. Marshall, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Filed Apr. 9, 1956, Ser. No. 577,024 6 Claims. (Cl. 154-227) The invention described herein relates to the encapsulation of articles and more particularly to an improved conductor and method for insulating same. The primary object of the invention is to provide a process for postorienting polyethylene terephthalate and thereafter applying it to a coated object for obtaining the `application of a uniform pressure on the coated material or in molding an object to a predetermined configuration.

The prior art discloses the basic concept of obtaining a tight iitting layer of material on an article, such as an insulating layer on an electrical conductor, and in obtaining a slight contracting force on an underlayer of material previously applied thereto. For example, a well known process employed in obtaining a compressive force by an overtaping of material, is to soak cotton tape in water, apply it to the coated article and during drying, the cotton bers shrink and contract to exert a small force on the coating material. The use of leather in lieu of cotton for this purpose is often resorted to because of its greater strength of recovery i.e., force created upon shrinking, when moisture is removed from the wrapping. Still another method employed in waterproofing ducts and the like, while still obtaining the equivalent of shrinkage, is to stretch rubber tape while applying it to the duct, and upon curing of the tape, the rubber forms a rm waterproof bond with the duct while simultaneously obtaining the benets of a compressive force resulting from the stretch-taping operation. Recent developments in the chemical field have resulted in the production of materials which will shrink slightly upon heating and the use of these materials for the purposes described above has been suggested. For example, irradiated polyethylene exhibits shrinking properties, as disclosed in the copending application of Paul A. Goodwin and John R. Stirrat, Serial No. 518,403, led lune 27, 1955, and assigned to the same assignee as this invention. The use of irradiated polyethylene for insulating dynamoelectric machine coils is also known, as disclosed and claimed in copending application of Harold Kitson, I r., Walter L. Marshall and Henry E. Mazanek, Serial No, 568,615, filed February 29, 1956, and also assigned to the same assignee as the present invention, but this material does not contain a force of recovery approaching that of polyethylene terephthalate described herein and therefore cannot be effectively used to carry out the above described objects.

However, the above-mentioned processes, with the exception of those cited in the copending applications, utilized in encapsulating articles, have certain drawbacks in that they cannot provide uid tight structures without unduly increasing the weight of the final product. Moreover, it is not possible by practicing these processes, to obtain the application of a compressive force on the coating material or on the article itself to any great degree because the outer tapings do not have a very large recovery force, that is, the force created by shrinkage upon curing of the material is not great. Furthermore, they are expensive.

A thermoplastic lm which can be obtained much more reasonably in cost than current coatings and which has superior qualities is polyethylene terephthalate. This material, which is sold under the name of Mylar by E. I. du Pont de Nemours and Company, Wilmington, Delaware, has exceptional strength and a relatively high point of flow although it is a thermoplastic material. The polyrice ethylene terephthalate usually obtained commercially is in fiber or sheet form, and for many applications, is available in thin iilms in the order of from about 0.1 to 20 mils in thickness. In addition, the film is in an oriented, that is, a stretched condition, which orientation may take place by stretching the lm at room temperature or at higher temperatures but below the fusion point of the polymer.

This material is a very highly oriented lm made by drawing the film biaxially as it comes from the melt. rI'he nal stretched lm is held in the stretched configuration while the molecular orientation is heat-set at a temperature near 220 C. This heat setting reduces the thermal shrinkage so that the full recovery of draw will not take place. After cooling, measurement of the tensile recovery force when the polyethylene terephthalate is heated through the temperature range of 25 C.-l50 C., reveals that there is a low order of recovery. In fact, the tensile recovery force decreases with increase in temperature so that the recovery force averages approximately 700-750 p.s.i. through the temperature range of 25 C.- C.

Unexpectedly however, I have found that if this oriented polyethylene terephthalate as obtained commercially, is additionally uni-axially stretched or post-oriented in the range of 3% to 24%, at room or elevated temperatures, and thereafter heated in the` range of 25 C. to 150 C. for one-half to one hour, there will be recovery of substantially all of this post-oriented elongation. If the material is restrained in the stressed or stretched condition at this temperature, i.e., 25 C.-150 C., it exerts a very strong restoring force.

In carrying out my invention, I utilize the newly discovered characteristics of polyethylene terephthalate when it is applied as an outer tape on a layer of insulating material previously provided on a conductor. Upon heating, the post-oriented polyethylene terephthalate creates a recovery force suicient to compress the insulating layers against one another and the conductor while simultaneously forming a rm bond therebetween. The post-oriented polyethylene terephthalate preferably is a sacrifice taping and after the conductor cools, it is removed.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. My invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawings in which:

FIGURE 1 is an end View of a conductor provided with an outer layer of insulating material;

FIGURE 2 is a side view of a conductor showing a layer of insulation and an outer layer of polyethylene terephthalate used in `applying `a compressive force to the insulation; and

FIGURE 3 is a side view of a pair of brazed conductors showing insulation terminating short of the brazing point.

Referring now to the drawings wherein like reference characters designate like or corresponding parts throughout the several views, there is shown in FIGURES 1 and 2 `a conductor or lead 10 provided with insulating material 12. After application of the insulating material 12, an outer layer of post-oriented polyethylene terephthalate 13 is applied over the insulating material and heated at a temperature hereinafter described to effect the application of Va compressive force on the insulating material positioned therebeneath.

As mentioned above, polyethylene terephthalate uniaXially drawn or post-oriented at certain temperatures and elongations show a great recovery force when subsequently heated. When the draw temperature is increased, independent of the other variables used, the temperature necessary for maximum recovery force increases and is (l) The cable was thoroughly cleaned to remove grease and other foreign substances deposited on the wire.

(2) The entire length of the cable was taped with various materials in the following manner:

accomplished by a corresponding increase in recovery 5 force. The following test results illustrate maximum (azerggltglltallyer of '001 commerclal polyethylene tersllhrelcovetry figg 1V rsus ,erilplelrlauire on. S.am1p11es (b) One half lap layer of .008" irradiated polyethylene; lgV/elnj ereisronafelnyefla Gilg ongma eng (c) One half lap layer of .005 dicumyl peroxide catal unen l s X lyzed polyethylene;

(d) One half lap layer of .008 irradiated polyethylene; TDraw Draw Percent Shrink Recovery followed by emp., Speed, Elonga- Temp., Force O. inches/sec. tion C. (Max), (e) Two half lap layers of post-or1ented polyethylene P'S'i' terephthalate having the properties described above.

1.05 12 89 21 500 l5 (3) The cable with its several layers of insulation was 11b? i gg gg subjected to 135 C. temperature for one hour in order 1,8 12 9e 4:80() to obtain fusing of the insulation materials and to effect lg l gg (15gg shrinkage of the post-oriented polyethylene terephthalate. 1:8 12 110 800 (4) The post-oriented polyethylene terephthalate was 1.103 i 33 20 then removed from the cable. 1.(15 24 110 31150 The post-oriented polyethylene terephthalate exerted an unusually large compressive force on the various layers In View of the large force of recovery inherent in post of insulation to Squeeze out air bubbles ElPPeariri bei'ween oriented polyethylene te-rephthalate, it is evident that it the .layers and compressed the insulation materials tightly has unlimited application in many different fields and for against each other and the Cable' A further function varied uses. In order that those skilled in the art may Served by the @liter layersof Polyethylene terephihalfate better understand how the present invention may be prac- Was that of ProVldlllg a Casing on the cable so that during ticed, the following examples are given by way of illustrallrmg the msulauon becalise 0f lts S'lfsealiiig Pf 013er tion and not by Way of limitation tles, formed a smooth laminated coating impervlous to high pressure liquids, effective in res1sting attack by chemi- Example 1 cal substances and displayed high dielectric properties. A first layer of insulating material consisting of l" by The cable insulated by the above-described process was 5 mil thiek tape of irradiated polyethylene of the type submerged in water under a pressure of 2500 p.s.i. at 80 disclosed by Elliott I. Lawton and Arthur M. Bueche in C for three Weeks and ille resistance Values dropped io e copending application Serial No. 324,552, iiled December Value rio loWer ili'eri los megolims Subsequerliio this 6, 1952, arid assigned to the Same assignee as this inverttest, the motor disclosed in the aforemention Kitson et al. tion, was applied to a 300 mil solid copper bar approxiapplication havlng leads insulated in the manner dismately 18 in length. This was followed by a second closed herein, has operated for more than seven months Wrap of the same material Wound m the opposite direcunder conditions equivalent to the test without failure. tion to provide a build up of 30 mils on the bar Two 40 Prior to using the above-described method of insulating Wraps of Polyethylene terephthalate, post oriented apa Cable, there Were DO 'knOWl effective means 0f insulating proximatelv 24% and of one mil thickness, Was applied leads 0r cables after they were insulated and installed in over the irradiated polyethylene with a i/s to 1/2 overlay ille machine arid ilieir eIldS braZed 'rogeiller- The eOrldiand the resulting structure heated through the temperaiiori of tlie leads subsequent io breZirig W38 similar i0 ture range of 25 C. to 150 C. for about 1/2 to l hour 45 that shown in FIGURE 3 in which the insulation 12 ter- The maximum compressive force applied to the irradimiiiaied sl'lori of the Point Where the leads Were bre-Zed ated polyethylene was 70 p.s.i. normal to the conductor. together, as indicated ei 14- Commoiily employed Voeu- Upon cooling of the bar, the polyethylene terephthalate UmPi'essilre Processes hereiofore employed Were 0f 110 was removerl Examination after Curing revealed that value because the -leads were not accessible. However, there were no air pockets or depressions in the irradiated Subsqueiit io ille discovery of ille Pr oeess disclosed here' polyethylene and it provided a very smooth, well-sealed in, solution of ille Problem of sealing suoli leads Was Surfaoe readily apparent. For example, in obtaining the water- Mam, different methods vvere followed m Wrapping the proof covering on the brazed leads as shown in FIGURE bar and the example cited above produced the best re- 3, insulating materiels 12 Were applied over The leads i0 Salta It was found that a consistent overlay Should be by following the process described above to arrive at a used in winding the irradiated polyethylene. The over- Wellsealed laminated Sirueiurelay preferably consists of two wraps of one mil post- Example 3 oriented polyethylene terephthalate and that both materials should be wrapped as evenly as possible. A11 18" solid Copper bar having Width and thickness dimensions set out in the table below, was Iwrapped with Example 2 a heat hardenable resin impregnated mica-paper mat tape A cable lead for a dynamoelectric machine of the type with Ian overlay of 1 mil post-oriented polyethylene terdisclosed in the above-cited Harold Kitson, Jr., et al. coephthalate. Subsequent to heating and curing, dielectric pending application, was insulated by carrying out the tests were made on the nished product to determine the following steps: effectiveness in using polyethylene terephthalatc.

Mica Mat Bar Build Break- V. /Mil Sample Tape Dim., Layers down,

Width, inches (M Lapped) kv. inches Flat Edge Flat Edge 1% Varny 4 .110 .080 26.0 234 325 .1% V2x1% 4 .118 .076 35.0 297 461 er irxlyr 2 .050 .030 11.7 234 300 1% UM 2 .066 .039 18.7 280 479 1% rx 1% 2l .066 .039 15.8 240 405 Since the force of recovery is so great when the postoriented polyethylene terephthalate is heated to effect shrinkage, care must be taken to assure that there are no indentations or depressions in the object covered because the post-oriented polyethylene terephthalate, upon shrinking, will cause the underlaye'r of material to gather in the depressions and thereby adversely affect its insulation qualities. In the event flat conductors are used, semielliptical members may be placed on the ilat sides of the conductor between the insulating material :and the polyethylene terephthalate so as to obtain a uniform application of force to the material.

In view of the above, it is apparent that the treatment of van insulated conductor by utilizing the recovery force inherent in post-oriented polyethylene terephthalate imparts desirable qualities to insulated materials -which they do not possess when used alone. In another example, a build of 48 mils of mica-mat epoxy resin was applied to a conductor followed by an overlay of 3 layers of halflapped polyethylene terephthalate. This sample was then molded at 155 C. and held at 15 kv. for one minute with breakdown inally occurring at 19 kv. The same conductor, coated with two layers of the same base coating to provide a build of 100 mils, without the use of an overlay of polyethylene terephthalate, was subjected to dielectric test and breakdown occurred at less than 7.5 kv. In comparing this with the results obtained in the samples cited above, it is evident that the application of polyethylene terephthalate aids in producing a conductor having properties non-existent in the prior art.

Obviously many modifications and variations of this invention are possible in light of the above teachings. For example, it is known that the improved polyethylene terephthalate is particularly adaptable for use in molding thermosetting resinous materials to desired shapes and the invention is especially useful in fields where a combination of pressure and temperature, within the eiective temperature range of the polyethylene terephthalate, are necessary in molding operations. It is to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The process of insulating an electrical conductor comprising applying a iirst wrap of heat hardenable resin impregnated mica-paper mat tape to a bar of electrically conductive material, wrapping an overlay of post-oriented polyethylene terephthalate on said mica-paper mat tape, subjecting said bar with its wrappings of material to a temperature corresponding to a temperature where maximum shrinkage is obtainable in said post-oriented polyethylene terephthalate, and removing said polyethylene terephthalate from said bar upon cooling.

2. The process of insulating an electrical conductor comprising applying la first coating of irradiated polyethylene to a bar of conductive material, adding an overlay of post-oriented polyethylene terephthalate on said irradiated polyethylene, heating said bar with its coatings at a temperature greater than 25 C. to effect shrinkage of said polyethylene terephthalate to thereby obtain a 6 smooth, air-free covering of irradiated polyethylene on said conductor.

3. The process of insulating an electrical conductor comprising the steps of applying a first wrap of electrical insulating material on a bar conductor having at least one flat side, positioning a mating flat side of a semielliptical member in contact with the tlat side of the insulated conductor, wrapping an overlayer of post-oriented polyethylene terephthalate over said insulating material and said semi-elliptical member, subjecting said bar with the materials wrapped thereon to a temperature between 25 C. and 150 C. to obtain shrinkage of said polyethylene terephthalate in a uni-axial direction with consequent application of pressure to the semi-elliptical member and said insulating material therebeneath thereby to provide a smooth air-free layer of insulation on said conductor, and removing said layer of polyethylene terephthalate.

4. The process of insulating an electrical conductor comprising the steps of applying a iirst layer of irradiated polyethylene on a bar of conductive material, applying an overlayer of post-oriented polyethylene terephthalate on said irradiated polyethylene, heating said bar with its insulating layers at -a temperature greater than 25 C. to obtain shrinkage of the polyethylene terephthalate in a uni-axial direction and in an amount corresponding to the degree of post-orientation imparted thereto for providing a smooth air-free covering of insulation of said conductor.

5. The process of encapsulating an object by utilizing the tensile recovery force inherent in post-oriented polyethylene terephthalate comprising the steps of applying a first layer of material on said object, applying a second layer comprising post-oriented polyethylene terephthalate over said first layer, heating said object with its encapsulating layers at a temperature between 25 C. and 150 C. -for a period of time suliicient to secure a degree of shrinkage of said polyethylene terephthalate in a uni-axial direction corresponding to the degree of post-orientation initially imparted thereto, and thereafter removing said polyethylene terephthalate.

6. 'The process according to claim 5 wherein said degree of shrinkage is within a range of 3% to 24%.

References Cited in the file of this patent UNITED STATES PATENTS 2,304,210 Scott et al. Dec. 8, 1942 2,556,295 Pace June 12, 1951 2,578,899 Pace Dec. 18, 1951 2,606,134 Sanders Aug. 5, 1952 2,735,970 Peck et al Feb. 21, 1956 2,836,744 Clawson May 27, 1958 FOREIGN PATENTS 606,752 Great Britain Aug. 19, 1948 OTHER REFERENCES Papers of the American Association for Textile Technology, -Inc.; Lanning, 'I'he General Properties of Mylar so Polyester Film, March 1954, pages 76-79. 

1. THE PROCESS OF INSULATING AN ELECTRICAL CONDUCTOR COMPRISING APPLYING A FIRST WRAP OF HEAT HARDENABLE RESIN IMPREGNATED MICA-PAPER MAT TAPE TO A BAR OF ELECTRICALLY CONDUCTIVE MATERIAL, WRAPPING AN OVERLAY OF POST-ORIENTED POLYETHYLENE TEREPHTHALATE ON SAID MICA-PAPER MAT TAPE, SUBJECTING SAID BAR WITH ITS WRAPPINGS OF MATERIAL TO A TEMPERATURE CORRESPONDING TO A TEMERATURE WHERE MAXIMUM SHRINKAGE IS OBTAINABLE IN SAID POST-ORIENTED POLYETHYLENE TEREPHTHALATE, AND REMOVING SAID POLYETHYLENE TEREPHTHALATE FROM SAID BAR UPON COOLING. 